The present invention relates to an optical multiplexing/demultiplexing device to multiplex or demultiplex an optical signal passing through optical fiber communication circuits, and especially to an optical multiplexing/demultiplexing device of the variable coupling type where an attenuated optical signal whose attenuation is variable can be output at its optical output port through its branch circuit.
Optical multiplexing/demultiplexing devices of the variable coupling type, which are used to multiplex or demultiplex optical signals in an optical fiber communication circuits, are conventionally those which are made of optical fibers scrubbed as shown in FIGS. 5(a) and (b).
FIG. 5(a) shows a cross-sectional view of the optical multiplexing/demultiplexing device cut across the optical axes of the built-in optical fibers, which is made of optical fibers scrubbed in accordance with techniques known by the person skilled in the art.
FIG. 5(b) shows a cross-sectional view of the optical multiplexing/demultiplexing device cut along the optical axes of the built-in optical fibers, which is made of optical fibers scrubbed in accordance with techniques known by the person skilled in the art.
The configuration of the conventional optical multiplexing/demultiplexing device will be explained to make the problems of this type of device clear.
V-grooves 23 and 24 are arranged on the inner surfaces of a pair of substrates 21 and 22 so that they can mate with each other at the centers of the pair of substrates 21 and 22, respectively. Optical fibers 25 and 26 are arranged within V-grooves 23 and 24 and are fastened thereby adhesives. Thereafter, the clads 29 and 30 of the optical fibers 25 and 26 are scrubbed off along the optical axes thereof together with the substrates 21 and 22 until cores 27 and 28 of the optical fibers 25 and 26 are just exposed. The scrubbed surfaces of the optical fibers 25 and 26 are contacted to make them mate with each other.
This configuration permits the optical power passing through one of the optical fibers to move to the other at a predetermined ratio in accordance with the evanescent effect.
It is well known by the person skilled in the art that the ratio of the optical power passing through a first optical fiber to that moved to a second optical fiber can easily be changed by finely adjusting the distance S between the core of the first optical fiber and that of the second optical fiber.
The means for changing the ratio of the optical power branching from the first optical fiber into the second optical fiber when the optical power is incident on the first optical fiber can be explained referring to FIG. 5(a). The conventional multiplexing/demultiplexing device of a variable coupling type is, as an example, such that substrate 22 is continuously shifted by means of a micrometer (not shown) with respect to substrate 21 in a direction perpendicular to the optical fiber axes so that the distance S from fiber core 27 to fiber core 28 can be finely adjusted.
Unless the guide for shifting substrate 22 works correctly, substrate 22 may move in a zigzag fashion. Thus it is very difficult for the operator to continuously move substrate 22 while keeping the pair of optical fibers precisely in parallel.
Assume that the scrubbed surfaces of the optical fibers are approximately 5 mm in length along the optical axes when SM10/125 single-mode optical fibers are used in the assembly, and that distance S measured from one of the optical fibers to the other is 12 .mu.m. We define the optical power branch ratio as the ratio of the optical power branching from the first optical fiber into the second optical fiber when the optical power is incident on the first optical fiber. The shift of distance from one of the optical fibers to the other is approximately 12 .mu.m so as to change the optical power branch ratio from 50-to-50 to 90-to-10. Thus, any zigzag motion is to be avoided as far as possible because it causes errors in the optical power branch ratio.
The guide should be precise and is elastic, and it is difficult to fabricate a guide of small size at low cost.
The objective of this invention is to provide a multiplexing/demultiplexing device of the variable coupling type which can be built to be compact at low cost, and wherein the optical power branch ratio is arbitrarily adjustable.